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GEOLOGY
Which forces drive North America?
G. H. R. Bokelmann, Department of Geophysics, Stanford University, Stanford, California 94305-2215, USA. Pages 1027-1030
Everybody (or almost everybody) agrees that the plates under our feet move. But why do they move, or in other words, which forces drive them? There are two main views on this question: that the plates are driven by forces acting from the side, that is, pulled by subducting slabs and pushed by ridge-oceanic ridges; or that the plates are riding (passively) on mantle convection cells, and thus are driven from below. Unfortunately it has been very difficult to distinguish these two sets of driving forces. This paper addresses the question based on the pattern of deformation within the plates, specifically the North American plate. Seismic anisotropy is a tool that allows us to do so. The orientation of anisotropy we find is indicative of forces acting from below being important. Thus it appears that the North American plate is driven largely by mantle convection, which drives the continent over a mantle downwelling. The consequence is that North America will settle over this downwelling, and its motion will stop.
Catastrophic extinction of Caribbean rudist bivalves at the Cretaceous-Tertiary boundary.
Thomas Steuber, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, 44801 Bochum, Germany; et al. Pages 999-1002.
Strontium isotope ratios (87-Sr/86-Sr) of shells of rudist bivalves provide, for the first time, precise numerical ages for the Late Cretaceous Titanosarcolites limestones of Jamaica. The data indicate that species-rich reefal associations of rudist bivalves, corals, and associated benthos that thrived in marine environments of the Caribbean persisted into the latest Maastrichtian (66–65 Ma). This finding contradicts the currently accepted hypothesis of stepwise extinction of rudist bivalves in the middle Maastrichtian, and provokes the scenario of a catastrophic extinction related to the impact of a large extraterrestrial object at the K-T boundary. Our data show that the global pattern of timing as well as the causes of the demise of the distinctive reefal ecosystems of late Mesozoic tropical environments must be reevaluated.
Venus: Timing and rates of geologic activity.
Alexander T. Basilvesky, Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow 117975, Russia and James W. Head, Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. Pages 1015-1018.
Venus is similar to Earth in many ways (size, density, and position in the solar system). Surprisingly, its surface geological record appears to date from the most recent few percent of solar system history, despite not displaying plate tectonic-like renewal activity at present. This synthesis and interpretation of the observed recent geological history of Venus suggests very high initial rates of endogenic (volcanic) activity, followed by more recent lower rates, and a presently relatively quiescent planet.
Observations and sampling of an ongoing subsurface eruption of Kavachi volcano, Solomon Islands, May 2000.
Edward T. Baker, Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way Northeast, Seattle, Washington 98115-6349, USA; et al. Pages 975-978.
On May 14, 2000, scientists from Australia, New Zealand, the US, and Papua New Guinea aboard the RV Franklin enjoyed an unexpected and spectacular encounter with an erupting subsurface volcano in the Solomon Islands. After weeks searching for hydrothermal discharge from many underwater volcanoes between Papua New Guinea and the Solomon Islands, the Franklin arrived at Kavachi, named for a legendary sea god of the region. The scientists found not a quiet seamount, but an active volcano violently erupting steam and lava every few minutes. The summit of Kavachi lies only a few meters below the sea surface and has undergone periods of eruption and subsidence in the past. But until this cruise, no attempt had been made at sampling during an eruption. Only a very few subsurface volcanoes anywhere in the ocean have been sampled during an eruption. The scientists were able to collect a unique set of seawater samples at a distance of less than 1.5 km around the summit. While dense plumes of volcanic ash were found throughout the water column down to >1000 m, the samples were almost devoid of volcanic tracers such as helium and dissolved metals. These findings indicate that even during an eruption the volcano flanks were relatively impermeable to fluid emissions and magmatic gasses from within the volcano itself. The most visible submarine effect of the eruption was physical rather than chemical--the mobilization of countless layers of lava shards from the volcano flanks, highlighting an under-appreciated mode of pyroclastic transport and dispersal.
Integrated chemostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica.
Gary S. Wilson, Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand; et al. Pages 1043-1046.
This article summarizes some important new results from a the multinational Cape Roberts Project that obtained three cores from up to 1000 m deep into the sea floor on the edge of Antarctica. One of the cores fortuitously recovered a thick interval of strata from the interval containing an important boundary (Oligocene-Miocene) in the geological record. The exact age of the Oligocene-Miocene boundary is currently debated, and suggested ages vary by more than a million years. The age of this boundary in the Cape Roberts project core was established very precisely at 24.0 million years using an integration of multiple dating methods. This age provides an important new calibration point for the geological time scale. It is particularly exciting that the new calibration point should be derived from Antarctica because dating of strata from Antarctica has proven historically difficult and this result flags a potential "coming of age" in our ability to date strata in Antarctica.
Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy.
C. Kevin Boyce, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA; et al. Pages 1039-1042.
The walls of the water-conducting cells of vascular plants possess several concentric layers, each no more than a few microns thick, made up of differing concentrations of the phenolic compound lignin and of carbohydrates such as cellulose. X-ray spectromicroscopy allows imaging of the spatial distribution of different types of organic chemistry at a submicron scale. We have used this emerging technique to demonstrate that multiple chemically distinct layers are preserved in individual cell walls from plant fossils as old as 400 million years (the oldest known anatomically preserved land plants) even though all of the original chemistry has been thoroughly altered over geologic time. From this we can draw several important conclusions. First, it verifies that organics derived from such labile material as carbohydrates can be preserved in fossils. Second, it indicates that all chemical alteration of the original biological materials happens in place at a submicron scale. Together, these points demonstrate that cellular biochemistry and physiology can be directly studied in fossils hundreds of millions of years old.
Rapid and widespread vegetation responses to past climate change in the North Atlantic region.
John W. Williams, National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, California 93101, USA; et al. Pages 971-974.
Studying biological responses to past abrupt climate changes provides insight into the resilience of modern ecosystems to future climate events. Lake sediments, which record (1) information about variations in terrestrial vegetation composition (from changes in the relative abundance of various pollen types), and (2) independent proxies for past climates (e.g., stable isotopes, aquatic fossil assemblages), are ideal for study because they minimize correlation uncertainty. From a synthesis of high-resolution sediment records from eastern Canada and Europe, the authors have shown that plant community composition responded within 0 to 200 years to the abrupt climate events accompanying the last deglaciation. Median response times were less than 100 years and were similarly rapid across continents, despite differences in topographic relief and species composition. These response times suggest that extant plant communities may already have begun to respond to recent climate changes, although these effects may be difficult to distinguish from other anthropogenic impacts.
Epeirogenic uplift above a detached slab in northern Central America.
Robert D. Rogers, Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, Texas 78759-8500, USA; et al. Pages 1031-1034.
Why are there mountains in Central America? To answer this question Rogers, Karason, and van der Hilst demonstrate that Central America experienced regional near-vertical uplift. They show from the distribution of elevation in tectonically inactive regions that half of elevation is concentrated between 800 m and 1000 m, representing a dissected plateau produced by the uplift. The uplift left a network of river meanders deeply entrenched in the bedrock from southern Nicaragua to southern Mexico. Using seismic tomography methods to image the mantle beneath Central America, they show that the mountains, entrenched meanders, and dissected plateau of northern Central America directly overlie a detached slab. Detachment and sinking of the slab allows an influx of hot, more buoyant mantle beneath Central America that caused the surface uplift from which erosion sculpted the mountains.
Microdiamonds in a negacrystic garnet websterite pod from Bardane on the island Fjortoft, western Norway: Evidence for diamond formation in mantle rocks during deep continental subduction.
Herman L. M. van Roermund, Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA, Utrecht, Netherlands; et al. Pages 959-962.
Micro-diamonds (5 um in size) have been discovered in an orthopyroxenite lens within a mantle-derived peridotite body from the Western Gneiss Region (WGR) of the Norwegian Caledonides. During the Caledonian orogeny (420–380 Ma) Baltica and Laurentia collided, resulting in the westward subduction of Baltica underneath Laurentia. During this subduction period, slices of the peridotite hanging-wall became incorporated into the subducted Baltica continental crust and infiltrated by subduction fluids with a predominant C-O-H composition. During ongoing subduction, reduction of the infiltrating fluids occurred, resulting in precipitation of carbonates and/or free carbon. When the subducted peridotite lens, now incorporated in continental crust, was metamorposed under ultra-high pressure (UHP) conditions the carbon transformed into diamond. During subsequent backward transport to Earth's surface (exhumation) the diamonds remained preserved only in those places where they were protected by strong container minerals i.e., inside spinels embedded within garnets. Elsewhere in the rock the micro-diamonds transformed back into graphite. Similar micro-diamond occurrences have been reported elsewhere in the world but the WGR example is so far unique because now, for the first time, diamond formed in a subduction zone during UHP metamorphic conditions, was found within mantle rocks and not within continental crust. This opens up new ways for the diamond industry.
GSA TODAY
New evidence for abrupt climate change in the Cretaceous and Paleogene: An Ocean Drilling Program expedition to Shatsky Rise, northwest Pacific.
Timothy J. Bralower, Isabella Premoli Silva, Mitchell J. Malone, and Scientific Participants of Leg 198
Ocean Drilling Program Leg 198 in August to October 2001 was designed to understand the causes, nature, and mechanics of "greenhouse" climates that characterized much of the Cretaceous and early Paleogene as well as of transient but critical climate events during this period. The aerial extent and importance of the Pacific in global circulation makes this ocean a critical target for investigation of warm climatic intervals; Shatsky Rise in the northwest Pacific was known to preserve high-quality Cretaceous and Paleogene sections. Leg 198 recovered an impressive 140 m.y. package of pelagic sediment at depths between 170 and 623 meters below the seafloor. The key success of the drilling was the abundant evidence for short-lived (<1 m.y.) warming events, and other major intervals of rapid climate and environmental change. These transient events, preliminary interpretation of which are described in this paper, include an interval in the early Aptian (120 Ma) associated with global oceanic anoxia, extinction events in the mid Maastrichtian (69 Ma) and at the Cretaceous-Tertiary (K-T) boundary (65 Ma), a biotic event in the late Paleocene (58.4 Ma) that may be associated with rapid climate change, an abrupt warming event at the Paleocene-Eocene boundary (55.5 Ma), and a rapid cooling step near the Eocene-Oligocene boundary (33.5 Ma) that brought final closure to the early Paleogene greenhouse climate interval. These transient events caused major upheaval in marine communities and profoundly altered biogeochemical cycling.
To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review the complete table of contents for the November issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=030&issue=11